(a) Field of the Invention
The present invention relates to an MMIC (Monolithic Microwave IC) incorporating an MMIC bare chip which is a high frequency semiconductor element.
(b) Description of the Related Art
FIG. 1 is a sectional view showing the structure of an example of conventional MMIC packages, and FIG. 2 is a perspective view showing the peripheral structure of MMIC excluding a cap. The conventional MMIC shown in these figures has a metal header 101 made of copper or copper-tungsten, an MMIC 102 die-bonded on the header, and a small plate 103 die-bonded onto the header 101 in the vicinity of a source terminal of the MMIC. The small plate 103 is prepared by plating the both surfaces of a ceramics plate acting as a substrate having a high dielectric constant, and functions as a capacitor formed between the opposing plated surfaces. The power supply from a power source to the MMIC 102, and the input and the output of high frequency signals are performed through external terminals (pins) 104 inserted into the metal header 101. The pins 104 and the MMIC 102 are generally connected by means of bonding wires 105 made of gold or the like. A capacitor for bypassing high frequency signals is inserted between the source line of the MMIC 102 and the GND (in this case, the metal header surface acts as a GND surface) by connecting the bonding wire between the source terminal of the MMIC 102 and the surface of the capacitor 103 and further by connecting a bonding wire between the surface of the capacitor 103 and the pin 104. The whole MMIC 102 is finally covered with a cap 106, which is mounted on the metal header 101 by means of seam welding or brazing.
Since, in this construction, the bonding wire having a diameter of 20 micrometers is employed for inputting the high frequency signals to the MMIC, the externally observed electrical characteristics, especially the input and output impedances, of the MMIC bare chip unit and of the MMIC packages differ from each other due to the impedance component of the wire itself. As a result, the impedance matching is required for obtaining high input and output power efficiencies. The impedance matching is, in general, carried out by means of the capacity component of the capacitor for compensating the inductance component of the wire. In this case, its characteristics may be deteriorated during the mounting of the MMIC on the package even if the frequency range of the MMIC is designed to be broad because the frequency range capable of conducting the impedance matching is narrowed.
Based on these reasons, in order not to deteriorate the characteristics of the MMIC as much as possible, the mounting must be so performed that the inductance component of the wire is reduced to as low as possible, and in the other words, that the bonding wire length is reduced to as short as possible. Accordingly, a so-called flip chip mounting method attracts attention in which a bump, in place of the wire, is formed on an electrode on the MMIC surface which is downward directed to mount the MMIC on the substrate by the bump, because the minimum interconnect length may be realized and the electric characteristics may be best utilized.
However, several problems arise for putting the above minimum connection length technique employing the bump to the practical use.
A first problem is that a bypass capacitor which depresses the impedance of the source line to a low level cannot be located just beneath the source terminal of the MMIC. Accordingly, the bypass capacitor is not mounted near the MMIC, which causes a high inductance component is generated between the bypass capacitor and the source terminal of the MMIC. Since this inductance component functions as an element for blocking a high frequency, it may disable to depress the impedance of the source line to a low level.
Since the interconnect length between the bare chip electrode and the wiring pad on the substrate is short in the flip chip mounting method, the stresses due to the difference between thermal expansion coefficients of materials and to shocks cannot be absorbed, which causes the MMIC to be brittle. Accordingly, the electric connection between the substrate and the bare chip is generally reinforced by filling the gap between the bare chip and the substrate with epoxy based sealing resin, so that the stress on the bump connecting portion is lessened to secure the reliability of the connecting portion. Since, in the case of the MMIC, the bare chip is designed for use in air having a specific inductive capacity of 1, the electrical characteristics may be altered due to the change of designed values if such a dielectric substance as epoxy resin is present around the bare chip. When the flip chip mounting method is applied as it is to an ordinarily designed bare chip, the resin sealing may not be performed depending on circumstances. This tendency appears more remarkably with the increase of a frequency employed.
In most cases, the ordinary MMIC is so designed that its circuit possesses a micro strip structure. Since, in this case, the rear surface of the bare chip is a GND surface, the GND surface becomes the top surface after the flip-chip mounting. As a result, the connection with the GND surface formed on the substrate mounting the bare chip becomes difficult. If the potential of the GND surface of the bare chip cannot be maintained the same as that of the GND surface of the mounting substrate, the electrical characteristics may be subject to adverse effects such as unnecessary resonance.